[linux-lvm] exposing snapshot block device

[Gionatan Danti]

On 23/10/19 12:46, Zdenek Kabelac wrote:
> Just few 'comments' - it's not really comparable - the efficiency of 
> thin-pool metadata outperforms old snapshot in BIG way (there is no 
> point to talk about snapshots that takes just couple of MiB)

Yes, this matches my experience.

> There is also BIG difference about the usage of old snapshot origin and 
> snapshot.
> 
> COW of old snapshot effectively cuts performance 1/2 if you write to 
> origin.

If used without non-volatile RAID controller, 1/2 is generous - I 
measured performance as low as 1/5 (with fat snapshot).

Talking about thin snapshot, an obvious performance optimization which 
seems to not be implemented is to skip reading source data when 
overwriting in larger-than-chunksize blocks.

For example, consider a completely filled 64k chunk thin volume (with 
thinpool having ample free space). Snapshotting it and writing a 4k 
block on origin will obviously cause a read of the original 64k chunk, 
an in-memory change of the 4k block and a write of the entire modified 
64k block to a new location. But writing, say, a 1 MB block should *not* 
cause the same read on source: after all, the read data will be 
immediately discarded, overwritten by the changed 1 MB block.

However, my testing shows that source chunks are always read, even when 
completely overwritten.

Am I missing something?

-- 
Danti Gionatan
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